Atmospheric humidity

Water vapor transport processes are driven by gradients, so atmospheric humidity measurements are critical inputs to models at a variety of scales. This chapter first gives a brief introduction on the role of humidity in the atmosphere. The primary metric for global climate change is mean global temperature, but atmospheric temperature and humidity are linked. The chapter contains a discussion of measured and expected trends in both mean global atmospheric humidity and changes in spatial distribution. Neither of the two fundamental measures of humidity-vapor pressure and mixing ratio-is directly and routinely measured with commercially available sensors or instruments. The chapter describes the most common types of instruments, such as psychrometers and dew cells, and how they are typically applied. It also briefly discusses the influence of humidity on the comfort levels of animals and on plant disease.